Electrical interlocks prevent elevator door opening during a run. In fact, safety devices will shut down the elevator if the door opens away from a stop. However, it may be desirable to begin opening the door in advance of stopping at a landing.
Safety codes define an outer door zone (ODZ) for a destination landing, at which point the doors are allowed to begin opening if the car is below a code-defined speed limit. Sensors at the outer door zone signal elevator cab presence thereat and are part of a bypass of safety devices inhibiting car motion with open doors. It will be assumed hereinafter that car speed and arrival at a destination appropriately constrain the bypass closure in addition to the proximity of the car to a floor.
An inner door zone (IDZ) is also defined, either less than or equal to the width of the outer door zone, at which point codes allow for slow velocity movement of the cab for leveling, with the doors open.
In the case of dissimilar inner and outer door zones, typical prior art door zone sensors employ two sensors for each door zone (IDZ and ODZ), a total of four sensors, since a single sensor failure cannot be allowed to bypass the safety devices. A typical outer door zone is =/- twelve inches (300 mm) from the landing, and a typical inner door zone is +/- three inches (75 mm) from the landing, for this case.